The two-volume set LNCS 4051 and LNCS 4052 constitutes the refereed proceedings of the 33rd International Colloquium on Automata, Languages and Programming, ICALP 2006, held in Venice, Italy, July 2006. In all, these volumes present more 100 papers and lectures. Volume I (4051) presents 61 revised full papers together with 1 invited lecture, focusing on algorithms, automata, complexity and games, on topics including graph theory, quantum computing, and more.